This invention relates generally to methods and apparatus for magnetizing a permanent magnet, and specifically to magnetizing a magnet used in a magnetic resonance imaging (MRI) system.
There are-various magnetic imaging systems which utilize permanent magnets. These systems include magnetic resonance imaging (MRI), magnetic resonance therapy (MRT) and nuclear magnetic resonance (NMR) systems. MRI systems are used to image a portion of a patient's body. MRT systems are generally smaller and are used to monitor the placement of a surgical instrument inside the patient's body. NMR systems are used to detect a signal from a material being imaged to determine the composition of the material.
These systems often utilize two or more permanent magnets directly attached to a support, frequently called a yoke. An imaging volume is providing between the magnets. A person or material is placed into an imaging volume and an image or signal is detected and then processed by a processor, such as a computer.
The prior art imaging systems also contain pole pieces and gradient coils adjacent to the imaging surface of the permanent magnets facing the imaging volume. The pole pieces are required to shape the magnetic field and to decrease or eliminate undesirable eddy currents which are created in the yoke and the imaging surface of the permanent magnets.
The permanent magnets used in the prior art imaging systems are frequently magnet assemblies or magnet bodies which consist of smaller permanent magnet blocks attached together by an adhesive. For example, the blocks are often square, rectangular or trapezoidal in shape. The permanent magnet body is assembled by attaching pre-magnetized blocks to each other with the adhesive. Great care is required in handling the magnetized blocks to avoid demagnetizing them. The assembled permanent magnet bodies comprising the permanent magnet blocks are then placed into an imaging system. For example, the permanent magnet bodies are attached to a yoke of an MRI system.
Since the permanent magnets are strongly attracted to iron, the permanent magnet bodies are attached to the yoke of the MRI system by a special robot or by sliding the permanent magnets along the portions of the yoke using a crank. If left unattached, the permanent magnets become flying missiles toward any iron object located nearby. Therefore, the standard manufacturing method of such imaging systems is complex and expensive because it requires a special robot and/or extreme precautions.
The prior art permanent magnet bodies often do not have an ideal shape for use in an MRI system because the blocks may have a somewhat imperfect shape and/or may not perfectly fit together. An improperly shaped permanent magnet has poor field homogeneity and requires the addition of a large number of shims to improve the field homogeneity.
Furthermore, the characteristics of the prior art permanent magnet bodies sometimes change unpredictably during the operation of the MRI system. For example, the magnetization of the prior art permanent magnets sometimes changes unpredictably during the application of gradient fields when the MRI is operating. Thus, the prior art permanent magnets have been known to partially demagnetize during the application of the gradient field pulses.
In order to magnetize the prior art permanent magnet, a pulsed magnetic field is used. Usually the pulse energy required to magnetize a permanent magnet is very high. For example, the pulsed magnets are energized with a capacitor bank or an external power supply (i.e., a wall power outlet) used with a magnetically operated switch, which provide a pulsed current to the pulsed magnet. Thus, the magnetization process requires an expensive, complicated and energy consuming power source which is capable of providing a pulsed magnetic field of a sufficient power.